Process for preparing a photographic emulsion



United States Patent 3,436,222 PROCESS FOR PREPARILIG A PHOTOGRAPHIC EMULSION Ramsis Gobi-an, St. Paul, and Ronald F. Ofstead, Maplewood, Minn., assignors to Minnesota Mining and Manuigcfuring Company, St. Paul, Minn., a corporation of e aware N0 Drawing. Filed Feb. 26, 1965, Ser. No. 435,676 Int. Cl. G03c I 72 US. Cl. 96-114 6 Claims This invention relates to photographic products and more particularly to photographic emulsions and lightsensitive silver halide-containing media, and to a process for the preparation of gelatin-free photographic emulsions and the like.

It is well known that natural gelatins are commonly employed as the protective colloid in photographic emulsions. These natural products have serious disadvantages which have brought about a search for substitutes for gelatin which are synthetic, and, while retaining the requisite properties of gel formation and protective colloid action, would be free from the objectionable shortcommgs.

Among the disadvantages of gelatin are variability in physical properties and chemical composition which lead to variations in the physical and photographic properties of the finished photographic emulsion; and variation in the susceptibility of the silver halides contained in these emulsions to sensitizing and desensitizing substances, developng agents and the like. Furthermore, gelatin products being brittle when dry, gelatin emulsion layers crack easily.

Much research work has been carried out hereinbefore in a search for a synthetic material which would be free from the disadvantages of gelatin for use in photographic applications, with constant physical and chemical constitution. Attempts have been made to lind a single synthetic material which would act as a protective colloid for precipitation of silver halide grains, and permit Ostwald ripening of these grains, which while being gellable, would allow chemcal sensitization of the emulsion, and in addition would have good physical and mechanical properties in tfilm form.

A number of materials have been developed which have the thermally reversible gelling properties of gelatin dispersions, and which can be used as gelling agents for photographic emulsions. A limited degree of success has been achieved with these materials. However, the synthetic materials heretofore proposed have had serious shortcomings when used in photographic emulsions. These involved one or more of the factors set forth above, as well as insuflicient light-sensitivity, high fog levels, poor storability, etc.

The art has also attempted to find alternate methods for precipitating siliver halide grains, so as to avoid the necessity for washing, etc. Thus quaternary alkyl ammonium halides have been employed for this purpose with apparent success. However, the necessity for rapid and essentially complete removal of the resulting ammonia or amine reaction products limts this approach.

It is an object of this invention to provide gelatin-free photographic emulsions of superior light-sensitivity.

Another object of the invention is to provide lightsensitive photographic materials employing silver halide in which combinations of synthetic polymers are used, each polymer performing a selected function in compatibility with the other polymers present and with improved results.

Another object of the invention is to provide a novel process for the production of photographic emulsions.

Yet another object of the invention is to provide a unique method for production of photosensitive silver halide grains.

A still further object of the invention is to provide photographic materials using the emulsions of the invention.

Other objects of the invention will be apparent from the disclosure hereinafter made.

In accordance with the above and other objects of the invention, it has been found that greatly improved photographic products can be produced using synthetic polymeric binders in the emulsion thereof, when a first synthetic polymeric material selected from halide salts of certain classes of polymers is employed as a source of halogen, and as a peptizing agent or protective colloid during precipitation of the silver halide, and subsequent Ostwald ripening of the silver halide grains and a second synthetic material is used as a film-forming medium to provide preselected physical and chemical characteristics. This combination provides highly advantageous flexibility in preparation of photographic emulsions and in the light-sensitive materials thus produced.

It has been found that when certain salts of polymeric polyamines are employed as a peptizing agent and source of halogen ion in the medium for the precipitation of silver halide, followed if desired by Ostwald ripening, silver halide grains of advantageous size and sensitivity are produced without the concomitant production of inorganic or organic reaction products which must he removed from the emulsion. To these grains, with or without removal of part or all of the protective colloid, is added a polymeric binder having the desired physical and chemical properties. The result is a greatly improved light-sensitve emulsion having wholly synthetic polymeric constituents which is suitable for all photographic uses. Thus photographic emulsions containing greater amounts of silver than is possible with other synthetic media are obtained. They have much lower fog level than the synthetic media heretofore available and are capable of being developed to high contrast. The sensitivity of the emulsions of the invention is extremely good, even without added sensitizers.

Broadly speaking, the synthetic polymeric material for precipitation of silver halide and Ostwald ripening thereof is chosen from a class of synthetic polymers comprising water-soluble polymeric polyamine halide salts which act as source of halide ion for the formation of silver halide, and which also function as a protective colloid for precipitation of silver halide, and which permit Ostwald ripening to occur. Examples of such polyamines are polyethylenimine, polypropylenimine and polyaminoalkyl acrylates. These polyamines, when neutralized with a hydrohalic acid, form amine salts, unless all of the valence bonds of the amino groups are attached to carbon and such polymers are treated with an alkyl or aryl halide, in which case a quaternary ammonium structure results. Polymeric polyamine salts and polymeric polyamine quaternary halides are included within the scope of the terms polymeric polyamine halide salts and halide salts of polymeric polyamine as used in the specification and claims of this application for Letters Patent.

The polymeric polyamines used in the invention can be further defined as containing from about 3 to 50 percent of amino nitrogen by weight, and preferably, from about 5 to 35 percent of nitrogen by weight. This corresponds broadly to an amino nitrogenzcarbon ratio of from about 1:1 to 1:30. Preferably the ratio is from about 1:2 to 1:20.

Polymers are known to consist of chains of atoms, the chains being of varying lengths. Thus, for example, when molecular weights of polymers are stated, they are average numbers which include lower and higher molecular Weight species. The polymeric polyamines employed in the present invention are those having molecular weight sufiicient to act as a protective colloid, and broadly speaking, they have molecular weight ranging from about 1000 up to 100,000 or higher, on the average. In terms of repeating units, however, they can be characterized as having from about to 2000 or more repeating units. Copolymers are also useful in the process of the invention, providing they meet these requirements, for example, copolymers of ethylenimine and propylenimine, and the like. Copolymers in which one of the monomers contains no amino nitrogen are also useful; e.g., copolymers of polyethylenimine and polyethylene oxide can be used within the limits set forth. Such copolymers, and mixtures of polymers, are included within the scope of the term polymeric polyamine as used herein.

Some polymers in their lower molecular weight ranges as specified herein are of low viscosity and have little protective colloid action. In this case such polymers can be supplemented by adding to the solution in which precipitation of silver halide is to take place, non-gelatin, watersoluble polymers other than the polymeric polyamines, to increase the protective colloid action.

The above-mentioned second synthetic material, which is used as a binder for the emulsion of the invention, is selected from a class of materials comprising synthetic or suitably modified (at least partially synthetic) naturally occurring polymers of vegetable origin which can be coated upon a supporting base from a water dispersion. These include water-soluble, film-forming polymers which can be crosslinked or otherwise rendered water-insoluble, or which form thermally reversible gels. Similarly, polymers of this type which form latices can be employed. They are selected according to their known characteristics, in order to provide optimum physical and mechanical characteristics in the final photographic products.

The film-forming binder cap, if desired, be present during precipitation of the halide, or can be added to the precipitated and ripened grains of silver halide after these are formed. At least a part of the polymeric polyamine protective colloid present during precipitation of the silver halide grains can have been removed, if desired, prior to or after addition of the binder.

In accordance with the present invention there is now provided an extremely versatile system for producing light-sensitive emulsions and photographic materials of the most varied characteristics, as required by end-use or other considerations. The diversity of end-products from this combination of synthetic materials is readily apparent since materials may now be selected in order to realize properties as desired for specific finished photographic products. For example, in specific applications combinations of more than one polymeric polyamine and more than one film-forming material can be advantageous, and this is readily accomplished using the process herein disclosed.

The synthetic polymeric polyamines employed in this invention may be treated (neutralized) with either a hydrohalic acid or with an organic halogen compound reactive towards amines, to form the polymeric polyamine halide salt. Such compounds include hydrochloric, hydrobromic and hydriodic acid, as well as methyl iodide, methyl bromide, ethyl iodide, and the like. An important characteristic of such a salt is believed to be the uniform array or spacing of halide ions along the polymer chain. This appears to aid in preventing clumping and to promote formation of uniform silver halide grains during precipitation.

In addition, it appears there may be some complexing of the silver ions by the polymeric amine. While applicants do not intend to be bound by theory, nevertheless the use of such polymeric polyamine halide salts as a source of halide ion for the formation of silver halide by reaction with a soluble silver salt is believed to exert an important influence upon the formation of uniform silver halide grains. Furthermore, no inorganic salts are formed in the precipitation process which must be removed by washing or the like, as in prior processes.

Chloride, bromide, iodide or combinations of these anions may be employed in this invention by choice of the appropriate hydrohalic acid, or reactive organic halogen compound used to quaternize the synthetic polymeric polyamine, in order to prepare silver halide photographic emulsions of desired silver halide composition.

The choice of film-forming synthetic binder for the photographic emulsion is determined by the physical and chemical properties desired in the ultimate photographic products. Thus a film-forming material or a combination of film-forming materials is selected which is compatible with the polymeric polyamine halide salt and which may be readily crosslinked or otherwise rendered water-insoluble. Preferably, the film remains in or is treated to place it in condition in which it is permeable to or swelled by the solvent of the developer. For example, in a photographic emulsion system in which the polymeric polyamine is polyethylenimine and the film-forming material is polyvinyl alcohol, crosslinking or insolubilization may be achieved by the use of agents known in the art, for example, as disclosed in United States Patents 3,113,026; 2,720,468; 2,671,022; 2,646,411 and 2,376,371. Other crosslinking or insolubilizing agents which can be employed are formaldehyde, mucochloric acid and tris-(laziridinyl)-phosphine oxide.

Other examples of such synthetic polymeric film-forming binders are hydroxyethyl cellulose, hydroxypropylcellulose, polyvinylprrolidone and the like.

While emphasis has been laid on the use of synthetic polymeric materials as binders hereinabove, the filmforming binder employed can be gelatin if desired.

The precipitation of silver halide grains may be effected in the presence of the quaternized or neutralized synthetic polymeric polyamine salt alone or in the presence of both the polymeric polyamine salt and the filmforming polymer, depending upon the choice of synthetic materials employed and upon the subsequent operations desired in the emulsion preparation.

Adjuvants for light-sensitive emulsions, such as sensitizers and sensitizing dyes, fog inhibitors, developing agents and the like, all of which are well-known to the art, can also be present in the emulsions of the invention. Crosslinking agents, surfactants or hardeners can likewise be employed in the usual way.

Broadly speaking, the process of the invention is the following: An aqueous mixture containing the polymeric polyamine halide salt (quaternized or neutralized) which may contain the film-forming polymer, for example, containing polyvinyl alcohol, and while maintaining the mixture containing the polyamine halide salt in the dark, is combined with soluble silver salt, e.g., silver nitrate, in desired amount with proper stirring to insure substantial homogeneity of the resulting emulsion. An excess of the polymer salt may be present, but generally the amount of halide salt used is not less than that which is stoichiometrically equivalent to the amount of soluble silver salt employed.

After formation of the silver halide by reaction with the halogen anions of the polymeric polyamine salt, the emulsion may be coated as such or it may be maintained at an elevated temperature, say 50-60 C., for Ostwald ripening.

It is an advantageous feature of the emulsions of the present invention that suflicient halide ion can be provided by the halide salt of the polymeric polyamine, so that Ostwald ripening can be accomplished.

The precipitation of silver halide and ripening can be accomplished in the absence of the film-forming binder if desired. Furthermore, if desired, substantially all of the polymeric polyamine can be removed following the formation of silver halide grains of desired size. In these cases, the film-forming binder is added separately, with agitation to insure a homogeneous dispersion of silver halide grains. Because no inorganic cations are present from the silver halide formation, washing of the emulsion is unnecessary.

The fluid emulsions thus prepared can be coated upon a flexible or rigid supporting base, for example, baryta paper, and dried in the dark at room temperature. Likewise, photographic taking films can be made using appropriate glass sheets or transparent plastic film base material instead of the paper. These photographic materials have high sensitivity and excellent photographic characteristics. When made with synthetic polymers they also have physical properties superior to those of emulsions prepared using gelatin. Light-sensitive materials prepared in this way can be exposed and developed in the usual manner using the common developing agents.

The following examples, in which all parts and percentages are by weight unless otherwise specified, will illustrate the compositions and the methods for their preparation.

EXAMPLE 1 Three solutions are prepared as follows:

(A) 100 parts of polyethylenimine having molecular weight of about 30-40,000 in 100 parts of distilled water.

(B) 8 parts of polyvinyl alcohol (e.g., obtained commercially as Elvanol 72-51) in 92 parts of distilled water.

(C) 5.9 N aqueous silver nitrate solution.

These may be prepared with the aid of heat if necessary. Subsequent operations in this example are carried out at 25 C.

Five parts of solution A are added to 62.5 parts of solution B. While stirring, the mixture is acidified to a pH between pH 2 and pH 7 by the addition of 48 percent hydrobromic acid, following the pH change with a glass electrode pH meter. To the above mixture is then added 0.3 part of boric acid in 2 parts of water, slowly and with good stirring. The remainder of the operations are carried out in the darkroom. Five parts of solution C are added with good stirring over a period of 30 seconds, and the mixture is thereafter stirred thoroughly for 5 minutes. One part of a 12 percent aqueous solution of a non-ionic surfactant (e.g., Polytergent J-300, a polyethoxylated higher alcohol, containing approximately 9 moles of ethylene oxide) is added and the mixture is then stirred an additional minute. The emulsion thus prepared is coated on baryta paper and on subbed polyester film. The coatings are air-dried. Thereafter, they are stored in lightproof containers.

On exposure to light, using an ordinary press-type camera, followed by standard development with metalhydroquinone developer and fixing, negatives having excellent quality are produced. These when printed on the coated baryta paper give black and white prints of excellent quality and gradation.

The preparation of the emulsion as set forth above, when repeated using polyethylenimine of molecular weight about 1000 and 100,000, respectively, gives similar results.

EXAMPLE 2 For comparative purposes, systems similar to those of the invention, that is, using polyethylenimine as in Example 1, but forming the silver halide grains in the conventional way, were prepared as follows:

Five grams of solution A are added to 62.5 grams of solution B. While stirring, the mixture is acidified to a pH of 3 with nitric acid. 5.4 grams of potassium bromide are added in the above mixture and, in the darkroom, 6.3 grams of silver nitrate in 10 m1. of water are added to the well-stirred polymer mixture. 0.30 gram of boric acid in 2 ml. of water are mixed in, and the emulsion thus prepared in coated on a support. When negatives so made were exposed, developed and printed as in Example 1,

the photographic prints obtained were of poor quality, possessing a high fog level and lacking in definition. Thus, it is seen that photographic emulsions prepared employing synthetic polymeric amines which have been quaternized with an acid other than a hydrohalic acid, and in which the halide ion for silver halide formation was introduced in the form of an alkali metal halide, yield photographic products unsuitable for practical photographic uses.

EXAMPLE 3 Following the procedure of Example 1, 10 parts of solution A are mixed well with 8 parts of 48 percent aqueous hydrobromic acid. To this is added 100 parts of 3 percent aqueous cellulose ether solution (Klucel G, available from the Hercules Powder Co.), with good mixing. The various mixing, coating and drying steps are carried out at 25 C. After preparation of the polymer mixture, further operations are conducted in a darkroom. Ten ml. of solution C are added to the well-stirred polymer solution. Five parts of 5 percent acetone solution of mucochloric acid, 3 parts of 37 percent aqueous formaldehyde solution and 3 parts of surfactant solution (as in Example 1) are then added with stirring. The emulsion thus prepared is coated upon baryta paper. After airdrying, the paper is stored in light-proof containers. Excellent photographic prints are made upon such printing paper.

EXAMPLE 4 Nine parts of a 50 percent aqueous solution of polyproplyenimine (M. W. about 20-40,000) are acidified to pH 4 with 48 percent hydrobromic acid. To this solution are added parts of a 10 percent aqueous solution of polyvinyl alcohol, with good stirring. Thereafter, all operations are conducted with the exclusion of light. A solution of 5.4 parts of solution C (Example 1) is added to the well-stirred polymer mixture. A water solution of a hardener for polyvinyl alcohol, and 4 parts of surfactant solution (as in Example 1) are added with thorough stirring. The emulsion thus prepared is coated upon baryta paper, and the coating is air-dried. Thereafter, the paper is stored in light-proof containers.

EXAMPLE 5 The procedure of Example 1 is employed, except that poly-N-hydroxyethyl ethylenimine is employed. A solution of 5 parts of poly-N-hydroxyethyl ethylenimine (M. W. ranging from about 1040,000), 50 percent solids in Water, is added to 62.5 parts of solution B. While stirring, the solution is acidified with 48 percent hydrobromic acid to a pH value between pH 2 and pH 7. To this solution is added slowly and with good stirring, an aqueous solution of a hardner. Thereafter, operations are carried out in the dark, and 4.7 parts of solution C (the calculated amount) and a small amount of an aqueous solution of a non-ionic surfactant are added with good stirring.

The emulsion thus prepared is coated on baryta paper and on subbed polyester film. These coatings were airdried and then stored in light-proof containers. When exposed in a standard camera, and developed using the normal photographic and fixing methods, excellent negatives and prints were obtained.

EXAMPLE 6 Ostwald ripening of silver halide grains by the process of the invention is shown as follows: The solutions described in Example 1 are employed. To parts of solution A are added a sufiicient amount of 48 percent hydrobromic acid to bring the solution to between about pH 2 and pH 7. In the dark, and while stiring, 5 pats of solution C are added over a period of about 60 seconds with rapid agitation. The stirring speed is then reduced to gentle agitation and the mixture is heated at 60 C. for 40 minutes. Samples of the mixture are removed after 10, 20, 30 and 40 minutes of ripening. Electron micrographs of these samples show octahedral silver bromide crystals with maximum diameters as follows:

Time (minutes): Maximum diameter (microns) EXAMPLE '7 The procedure of Example 1 is followed except that solution A is not used. Instead, 12 parts of a polymeric poly quaternary amine bromide, made by completely quaternizing poly-4-vinylpyridine (M.W. about 5-40,000) by reaction with ethyl alpha-bromoacetate, are added to 62.5 parts of solution B. The addition of a small amount of 48 percent hydrobromic acid suffices to acidify the mixture. The process thereafter follows Example 1. Paper coated with the emulsion thus prepared can be exposed to light through a negative and developed with the ordinary metol-hydroquinone developing agents.

What is claimed is:

1. A method for preparing a photographic emulsion containing silver halide crystals which comprises preparing an aqueous solution of a halide salt of a polymeric polyamine having a molecular weight greater than about 1000 and containing about 3 to percent by Weight of amino nitrogen, and adding thereto an aqueous solution of a soluble silver salt.

2. The method of claim 1 in which said polymeric polyamine is polyethylenimine.

3. The method of claim 1 in which said polymeric polyamine is poly(N-hydroxyethyl) ethylenimine.

4. The method of claim 1 in which said polymeric polyamine is polypropylenimine.

5. The method of claim 1 in which said photographic emulsion is subsequently Ostwald ripened.

6. The method of claim 1 in which an at least partially synthetic polymeric, film-forming binder is added to said photographic emulsion and the photographic emulsion is free of gelatin.

References Cited UNITED STATES PATENTS 2,484,423 10/1949 Reynolds et al. 96l14 2,949,442 8/1960 Clavier et al. 96-l 14 3,084,132 4/ 1963 Shashoua 96l 14 OTHER REFERENCES Evva, F.: C. A., vol. 51, p. 16l58d, 1957. Evva, F.: C. A.," vol. 52, p. 180380, 1958.

NORMAN G. TORCHIN, Primary Examiner.

R. H. SMITH, Assistant Examiner.

US. Cl. X.R. 

1. A METHOD FOR PREPARING A PHOTOGRAPHIC EMULSIONL CONTAINING SILVER HALIDE CRYSTALS WHICH COMPRISES PREPARING AN AQUEOUS SOLUTION OF A HALIDE SALT OF A POLYMERIC POLYAMINE HAVING A MOLECULAR WEIGHT GREATER THAN ABOUT 1000 AND CONTAINING ABOUT 3 TO 50 PERCENT BY WEIGHT OF AMINO NITROGEN, AND ADDING THERETO AN AQUEOUS SOLUTION OF A SOLUBLE SILVER SALT. 